For purposes of this application the term "conveyor" is used generally and includes both conveyors and feeders, or any other apparatus that conveys or delivers material.
Vibratory conveyors use a vibrating trough to move material. Vibratory conveyors are often used in industry to meter the flow of bulk material, or to convey bulk material from one point in a process to another point. For example, in the snack food industry, vibratory conveyors are used to meter snack foods such as potato chips, popcorn, corn chips and the like from storage bins; or to convey and distribute snack foods from the cooking processes to the packaging machines. Such vibratory conveyors are generally of the resonant, two mass design to minimize operating power, and to provide a means to isolate the dynamic operating forces being generated from the support structure of the respective conveyor and any other surrounding equipment.
A vibratory conveyor of this type may include a base member supported by isolating spring members attached to a support structure; and a trough member, which feeds or conveys material. The isolating spring members and the base member adapt the conveyor to the physical height positions required for the conveyor to be integrated into the process flow. In some designs, the trough member, rather than the base member, is supported by the isolating spring members.
The base member is connected to the trough member by springs arranged with their longitudinal axes at a pre-selected angle to the vertical, thus imparting a directed force to the material during operation and causing the material to convey along the trough member.
A drive mechanism or "driver" is also connected between the base and trough members to cause the members to vibrate or move back and forth relative to one another. The drive mechanism may be an electromagnetic system with the electromagnet core connected to the base member and the electromagnet armature connected to trough member, or a motor and crank arm system, or a motor driven rotating eccentric weight exciter, or some other similar drive system. An example of an electromagnetic driver is disclosed in U.S. Pat. No. 5,293,987.
The natural frequency of the conveyor's mass and spring system (i.e., the frequency at which they would freely vibrate if struck by a hammer) is set to be close to its operating frequency or "speed" to take advantage of the phenomenon of "resonance." At resonance, energy stored in the spring system as it operates is returned to be "in phase" with the applied driving force, reducing the power required to operate the conveyor to about 25% or less of what would be required in a direct drive system.
The relationship of the natural frequency of a linear two mass vibratory system to the combined weight of the base and trough, and to the spring rate, may be expressed by equation (1): ##EQU1##
where:
N.sub.o =The natural frequency in cycles per minute, PA1 K.sub.d =The dynamic spring rate in lbs. per in. of deflection, PA1 g=The acceleration due to gravity in inches per sec.sup.2, and PA1 W.sub.R =The resultant or effective weight of the conveyor system, ##EQU2## PA1 W.sub.t =The weight of the conveyor trough in lbs. PA1 W.sub.b =The weight of the conveyor base in lbs. PA1 N=The operating frequency in cycles per minute, and PA1 N=The natural frequency in cycles per minute.
and where:
In practical linear two mass vibratory system designs, the ratio of the operating frequency to the natural frequency, sometimes indicated by the Greek letter Lambda (.lambda.), defined by equation (2), usually is set by the design parameters to lie within the range of 0.8 to 1.2. Generally, the closer .lambda. approaches 1, the greater the amplitude of the conveyor for a given driving force. ##EQU3##
where:
A .lambda. value of 1 indicates the conveyor would be operating exactly at resonance. A .lambda. value less than 1 indicates the conveyor would be operating sub-resonant (i.e., the natural frequency is greater than the operating frequency). A .lambda. value greater than 1 indicates the conveyor would be operating super-resonant (i.e. the natural frequency is less than the operating frequency).
Operation exactly at resonance is usually avoided since the system would be highly sensitive and erratic, involving the possibility of highly destructive vibration amplitudes in lightly damped systems. In applications involving heavy trough loads, it may be desirous to use a sub-resonant conveyor design because such design is less sensitive to changes in vibration amplitude with changes in load. For example, since an incremental increase in the load can effectively increase the trough mass, the natural frequency would decrease. Thus, applying equation (1), .lambda. increases, moving closer to 1 (resonance). The move toward resonance offsets the loss in amplitude caused by the damping effect of the increased load.
In applications involving light trough loads, it may be desirous to use a super-resonant conveyor design because such a design requires less spring rate and is therefore less costly. Also, such conveyors are more reliable to use in some applications because the material feed rate decreases with increasing material load, thus preventing equipment damage and possible downstream material flooding. For example, since an increase in the material load effectively increases the trough mass, the natural frequency would decrease. Thus, applying equation (1), .lambda. increases, moving farther away from 1 (resonance). The movement away from resonance further decreases amplitude already decreased by the damping effect of the load.
Therefore, it is advantageous to be able to operate a conveyor at a pre-selected ratio .lambda. of its operating frequency to its natural frequency. In order to operate the conveyor at the pre-selected ratio .lambda., compensation for varying parameters may be required. The natural frequency of the conveyor is dependent on the conveyor's spring coefficient and the spring coefficient can change over time. If some form of polymer spring is used, such as natural or synthetic rubber, the spring rate can increase with use, as the polymer continues to cure or age. Also, the natural frequency of the conveyor is dependent on the operating weight of the conveyor. The operating weight of the conveyor can be different than the original design weight due to variations within manufacturing tolerance, or modifications to the conveyor in the field.
Furthermore, the amplitude of vibration is affected not only by .lambda., but also by the driver's available power to deflect the spring/mass system, and to overcome the various system losses due to friction and damping. The driver's available power can be limited. In many conveyor designs, the operating frequency is fixed by a relationship to the power line frequency or is fixed to a fixed speed motor, or the like. The available power is also usually fixed as pre-established by the design limitations for a given model. Manufacturing and material tolerances, abrasive wear, and sometimes unauthorized modifications to the trough or base members in the field, reduce available power and have a detrimental effect on the trough amplitude. Given limitations in available power, it is important to optimize conveyor stroke amplitudes.
The present inventor has recognized that it would be desirable to provide an effective apparatus to adjust or "tune" conveyor parameters to obtain or to maintain the desired trough amplitude given the driver's available power. The present inventor has recognized that it would be desirable to provide an effective apparatus to tune conveyor parameters to offset changes in conveyor component weights or changes in spring coefficients.